The various embodiments relate to ultrasonic flow meters and, more particularly, to transducer assemblies employed in ultrasonic flow meters.
After hydrocarbons have been removed from the ground, the fluid stream (either in a liquid phase or a gaseous phase) is transported from place to place via pipelines. It is desirable to know with accuracy the amount of fluid flowing in the stream, and particular accuracy is demanded when the fluid is changing hands, or during “custody transfer.” Even where custody transfer is not taking place, however, measurement accuracy is desirable, and in these situations ultrasonic flow meters may be used.
An ultrasonic flow meter includes two or more transducer assemblies, each secured inside of a port in the body, or spool piece, of the flow meter. To contain the transported fluid within the flow meter, an end connector is secured over the exterior end of each transducer port in the spool piece. Thus, the spool piece and end connectors create a pressure boundary that contains fluid flowing through the meter.
To measure fluid flow through the meter, a first and a second transducer assembly is each positioned in a port in the spool piece, such that each transducer assembly faces the other. Each transducer assembly includes a piezoelectric element. When an alternating current is applied to the piezoelectric element of the first transducer assembly, the piezoelectric element responds by radiating an ultrasonic wave in the fluid being transported through the flow meter. When the wave is incident upon the piezoelectric element of the second transducer assembly, the second transducer assembly responds by generating an electric signal. Some time later, an alternating current is applied to the piezoelectric element of the second transducer assembly, and the piezoelectric element responds by radiating an ultrasonic wave through the fluid in the flow meter. When the wave is incident upon the piezoelectric element of the first transducer assembly, the first transducer assembly responds by generating an electric signal. In this way, the transducer assemblies transmit and receive signals back-and-forth across the fluid stream.
Each transducer assembly is connected to a cable that extends through the end connector to a location external to the spool piece, such as an electronics base enclosure typically mounted to the exterior of the spool piece. The cable carries the signals created by the piezoelectric elements to an acquisition board positioned within the electronics base enclosure, where the signal may be processed and subsequently used to determine the fluid flow rate through the meter.
In most conventional transducer assemblies, the piezoelectric element is positioned in one end of the transducer assembly proximal the fluid stream flowing through the spool piece. Typically, the piezoelectric element is positioned in a housing and surrounded by a matching layer that provides acoustical coupling between the piezoelectric element and fluid flowing through the spool piece. To optimize the quality of the ultrasonic signal (e.g., larger amplitude and faster rise time), the piezoelectric element is radially centered in the housing and the thickness of the matching layer between the piezoelectric element and the end of the transducer assembly in the fluid flow is carefully controlled. Specifically, radially centering the piezoelectric element insures that the ultrasonic wave is symmetrical about the transducer center which improves flow measurement accuracy because dimensional measurements of the port position in the meter bore typically assumes that the ultrasonic wave is in the center of the port hole. Further, radially centering the piezoelectric element eliminates concerns with the rotational orientation of the transducer in the port.
To properly position the piezoelectric element during manufacture of the transducer assembly, the piezoelectric element is typically positioned and held at the desired in the housing with a positioning tool. While holding the piezoelectric element at the desired location within the housing with the positioning tool, a first matching layer fill is disposed in the housing around a portion of the piezoelectric element. Without interfering with the matching layer, the positioning tool continues to hold the piezoelectric element in the desired position as the first matching layer fill solidifies and cures. Once the first matching layer fill has sufficiently hardened, it helps hold the piezoelectric element in place, and thus, the positioning tool may be removed before a second matching layer fill is disposed in the housing around the remainder of the piezoelectric element. Thus, during the manufacture of many conventional transducer assemblies, the proper positioning of the piezoelectric element within the housing is achieved with a specialized positioning tool, and further, a relatively labor intensive and time consuming two separate matching layer fill process is employed.
Accordingly, there remains a need in the art for transducer assemblies having piezoelectric elements properly positioned to optimize the quality of the ultrasonic signals. Such transducer assemblies would be particularly well received if their manufacture was relatively simple, low cost, and less time consuming.